Accurate collection of geophysical property data is a key to successful exploration and production of petroleum resources. Based on data such as electrical and nuclear properties collected in a well-bore, as well as the propagation of sound through a formation, geophysicists make an analysis useful in making many important operational decisions. The analysis includes determination of whether a well is likely to produce hydrocarbons, whether to drill additional wells in the vicinity of an existing well, and whether to abandon a well as being unproductive. Geophysicists may also use well-bore data to select where to set casing in a well and to decide on how to perforate a well to stimulate hydrocarbon flow. One method of collecting well-bore geophysical properties is by way of wireline well-logging. In wireline well-logging, a well-logging tool (also often referred to as a sonde) is lowered into a well-bore on an electrical cable, the wireline. The well-logging tool is an electrically powered measurement device that may, for example, collect electrical data, sonic waveforms that are propagated through the surrounding formation, or radioactivity counts. These measurements are usually converted to a digital form and transmitted on the wireline.
Other methods of collecting well-logging data are known as logging while drilling (LWD) and measuring while drilling (MWD). In these types of well-logging data is collected during the drilling operation.
Well-logging data is normally indexed by the depth at which the measurement was taken. However, with modern equipment the logging measurements may be taken on a time interval rather than on a depth interval and indexed against a time stamp. Such time domain logs are ultimately translated to depth index.
The accuracy of the data is a crucial element in the value of the analysis the well-log data and the correctness of decisions made based on that data. Data accuracy depends both on the accuracy of the measurements made and on the accuracy of the depth index.
In wireline logging the depth index is usually determined by measuring how much cable has been lowered into the borehole and measuring how much cable has been reeled back in. These measurements are done at the surface as the cable passes through the Integrated Depth Wheel (IDW).
IDW measurements are prone to inaccuracies with respect to the actual depth of the measurement tools. For example, the tools may become stuck in the borehole due to various borehole conditions. If the well is being logged in an uphole direction, which is most common, if the tool becomes stuck the cable may stretch as the winch continues to reel in the cable. While a wireline cable may seem very solid, if it is reeled out to several thousand feet, it becomes relatively elastic. Thus, the IDW may measure several turns while the measurement tool is in fact stuck. Conversely, when the tool works itself loose, perhaps from the tension applied through the cable, from its stuck position, the tool may overshoot its natural resting position. If the tool does overshoot the recorded IDW position, the IDW is again inaccurate.
Similar problems also occur in LWD and MWD logging, for example, coil tubing and drill pipe may become bent.
Several different approaches have been suggested for adjusting the recorded depth index. One approach described in Howard, Jr. et al. (U.S. Pat. No. 5,019,978) uses accelerometers to determine the location of the measurement tools. The accelerometer data is applied in an algorithm to correct the depth index. In another approach described in Method and Apparatus for Correcting the Depth Index for Well-Log Data, U.S. patent application Ser. No. 09/975,234, filed Oct. 11, 2001, to Ronald C. Kelly and co-assigned hereto, uses tension meter data in combination with the accelerometer data in a proportional-integral-derivative control loop algorithm to correct the depth index. While both the Howard, Jr. et al. and Kelly approaches may be useful in many circumstances there are drawbacks to each. For example, the Howard, Jr. et al. method requires accelerometer data. Not all tools have accelerometers. Also, in certain operations the tool motion may be so small that accelerometer data is not applicable. Kelly's approach requires tension measurements which also may not always be available.
Therefore, there is still a need for improved and alternative approaches to correcting the depth index during well-logging operations.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.